Variable ferromagnetic attenuator having a constant phase shift for a range of wave attenuation



May 2, 1967 PHASE SHIFT FOR Filed May 7, 1965 nlNHh-ruAN Ncso VARIABLEFERROMAGNETIC ATTENUATOR HAVING A CONSTANT A RANGE OF WAVE ATTENUATION 2Sheets-Sheet 1 .AS/GNAL CURRENT /42 SOURCE CURRENT SOURCE IN1/avro@ D.NGO

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ATTORNEY May 2, 1967 D|NH-TUAN NGO 3,317,863 VARIABLE FERROMAGNETICATTENUATOR HAVING A CONSTANT PHASE SHIFT FOR A RANGE 0F WAVE ATTENUATIONI Filed May 7, 1965 2 Sheets-Sheet 2 /l20 PERME/L/TV United StatesPatent O VARIABLE FERROMAGNETIC ATTENUATOR HAVING A CONSTANT PHASE SHIFTFOR A RANGE F WAVE A'ITENUATION Dinh-Tuan Ngo, Somerset, NJ., assignorto Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Filed May 7, 1965, Ser. No. 454,098 16 Claims.(Cl. S33-81) This invention relates to electromagnetic wave propagatingarrangements and, more specifically, to a circuit combination which ischaracterized by an electronically variable wave attenuation without anyattendant variable phase perturbation. r

A plurality of electronic wave transmission embodiments, such as coaxialcables, waveguides, strip lines and the like, are employed in highfrequency systems to propagate wave energy. To effect Various desiredcircuit operations in the high frequency spectrum'of interest, prior arttransmission structures have been loaded with ferromagnetic and/ ordielectric materials to employ the bulk properties of these substances..In addition, an external magnetic field has been utilized to bias theferromagnetic material included in such a structure to a particularvalue of permeability.

In particular, the ferroresonant effect exhibited by suitably biasedferrite elements has heretofore been employed in waveguides or the liketo partially absorb, and thereby attenuate, a wave translatingtherethrough. However, attendant to the attenuation produced by suchprior art arrangements, there is associated an undesirable shifting ofphase ofthe propagating wave. Where variable attenuating embodiments areutilized, the appurtenant variable wave phase shift has been foundparticularly objectionable.

It is therefore an object of the present invention to provide animproved electromagnetic wave attenuating arrangement.

More specifically, an object of the present invention is the provisionof a wave attenuating arrangement which produces a constant phase shiftfor a range of wave attenuation.

It is another object of the present invention to provide a waveattenuating embodiment which may be relatively simply and inexpensivelyfabricated, and which is highly reliable.

These and other objects of the present invention are realized in aspecific, illustrative electronically variable electromagnetic waveattenuating embodiment which does not alter the relative phasing of apropagating wave. The arrangement includes a wave transmission structureloaded with two ferromagnetic thin film elements characterized by apermeability with field-sensitive real and imaginary components. l

A first winding is coupled to the two thin film elements in a like senseto quiescently bias these elements to a zero relative phase shift,maximum attenuation point on their respective permeabilitycharacteristics. In addition, a signal winding is coupled in an oppositepolarity relationship to the two thin film elements to selectivelyreduce the wave absorption properties thereof. As the operating pointsfor the two thin film elements are symmetrically displaced about theirquiescent condition under the action of the signal winding, theseelements generate canceling phase perturbations for any given waveattenation.

It is thus a feature of the present invention that an electromagneticwave attenuating arrangement include a wave transmission embodimentloaded with two ferromagnetic thin film elements, magnetizing fieldbiasing circuitry coupled to the two film elements in a like sense, andsignal magnetizing field circuitry coupled to the two thin film elementsin an opposite polarity relationship.

ICC

It is another feature of the present invention that an electromagneticwave attenuator include a wave propagating structure loaded with firstand second ferroresonant elements characterized by a magneticfield-sensitive permeability which includes a wave absorbing imaginarycomponent and a phase shifting real component, constant magnetic fieldcircuitry -for biasing each element to ferroresonance, and magneticsignal source circuitry for supplying magnetic fields of an equalamplitude and an opposite polarity to the ferroresonant elements.

A complete understanding of the present invention and of the above andother features, ladvantges and variations thereof may be gained from aconsideration of the following detailed description of an illustrativeembodiment thereof presented hereinbelow in conjunct-ion with theaccompanying drawing, in which:

FIG. l is a schematic diagram of an illustrative electromagnetic waveattenuating arrangement made in accordance with the principles of thepresent invention;

FIG. lA is a cross-sectional diagram of a wave propagating structure 10included in FIG. l; and

FIG. 2 illustrates the relationship between permeability and the appliedmagnetic field for a plurality of ferromagnetic thin film elements 14and 15 included in FIG. 1.

Referring now to FIG. l, there is shown a specific illustrativeelectronically varia'ble electromagnetic wave attenuating arrangementwhich does not affect the relative phasi-ng of an associated translatingwave. The arrangement includes a wave propagating structure 10comprising a center conducting sheet 11 and two grounded conductingplanes 12 respectively disposed on either side of the sheet 11. yBetweenthe center conductor 11 and each of the grounded planes 12 there areinterposed two ferromagnetic thin film elements 14 and 15, and adielectric material 16 such as glass. The particular organization of thepropagating structure 10, which is known as a strip line, is illustratedin cross-sectional form in FIG. 1A.

An input wave source 20 is included in the FIG. l arrangement to supplyelectromagnetic wave energy to the input end of the strip line 10 via acoaxial cable 21. Similarly, a coaxial cable 26 is employed to connectthe output end of the line 10` to au output utilization means 25.

A biasing winding 30 is inductively coupled to the film elements 14 and15 in a like sense and connected to a biasing current source 32. Also, asignal winding 40, connected to a signal current source 42, is coupledto the film elements 14 in the same polarity as the biasing winding 30,and further inductively linked with a like number of turns to the filmelements 15 in a polarity opposite to the biasing winding 30. It isnoted that the windings 30 and 40 are only partially illustrated in FIG,l to preserve the clarity'thereof.

The ferromagnetic thin film elements 14 and 15 are characterized by afrequency sensitive permeability ,a which includes a bipolar realcomponent ,u and an imaginary component a, as shown in FIG. 2, whereinwhere j is the operator 1, with the magnitude of ,u being given by Theimaginary component p of the composite film permeability ,u is a narrow,peaked curve which is symmetrical about a magnetic field H0 at which thefilm elements 14 and 15 are ferroresonant. The real permeabilitycomponent ,uf is zero at the ferroresonant field Hg, and characterized'by symmetrically shaped positive and negative portions respectivelydisposed to the right and left thereof.

When employed in conjunction with the various wellknown field and wavetranslation equations which characterize wave propagation in the regionsbetween the center conductor 11 and the grounded conductors 12, the filmpermeability pi is multiplied by jw, where w is the angular frequency ofan incident wave of interest. Hence, the unipolar imaginary permeabilityportion it produces wave attenuation, and is designated the absorptionpermeability component. Correspondingly, the real permeability component,u/ is essentially reactive in nature (capacitive to the right of H;inductive to the left thereof), and shifts the phase of a wave travelingthrough the film medium.

In overall qualitative terms, the biasing winding 30 and biasing source32 are adapted to supply a magnetizing field of amplitude Ho to thefilms 14 and 15, thereby quiescently biasing these elements to a point100 of zero real permeability, and to the peak point 120 on theimaginary film permeability component p. With these conditionsobtaining, an input wave suppled by the FIG. l input source 20 to theoutput utilization means 25 via the line will undergo maximumabsorption, or attenuation, While not being subject to any relativephase shift since the film reactive permeability component is zero. Itis noted that the term relative phase shift, or more simply phase shift,as used herein refers to any shifting of the phase of a propagating waveother than the phase la-g necessarily associated with the transit timeof electromagnetic wave traveling through a given length of the FIG. lstrip line 10.

Responsive to a signal current supplied thereto by the associated source42, the signal winding 40 supplies a lmagnetizing field AH to the filmelements 14 in the same sense as the applied bias field HD, and alsosupplies a like amplitude field AH to the films 15 in a sense oppositeto the bias field Ho, as shown in FIG. 2. Specifically, the energizedsignal winding 40 drives the film elements 14 to points 101 and 121 onthe corresponding real and imaginary film permeability characteristics,while the film permeability operating points attain the correspondingvalues shown therefor by the points 102 and 122 in FIG. 2. `Because ofthe above-noted symmetry of the FIG. 2 curves, the imaginarypermeability component values at the points 121 and 122 are equal insign and magnitude, while the real component values at the points 101and 102 are equal in amplitude, but opposite in sign.

With the above magnetizing field conditions prevailing, the absorptionpermeability components given by points 121 (films 14) and 122 (films15) are less than the corresponding quantity 120 which characterized thefilms 14 and 15 when only the bias field Ho was present. Accordinglyeach of the films 14 and 15 absorbs relatively less power from theincident propagating wave and, therefore, the wave attenuation effectedby the loaded strip line 10 is reduced below its quiescent value.

However, since the films 14 and 15 real permeability component valuesrespectively indicated by the points 101 and 102 correspond toequal-valued capacitive and inductive reactances, the incident wavesupplied to the line 10 by the source 20 is delayed in phase by the filmelements 14, and advanced in -phase in an equal amount by the filmelements 15. Hence, the propagating wave translates 4through the FIG. lattenuating structure in precisely the same phase relationship whetheror not a signal field AH is present to displace the film operatingpoints from their quiescent values at the points 100 and 120 shown inFIG. 2.

Generalizing, it is observed from the above that the FIG. 1 arrangementwill produce more or less wave attenuation as the signal magnetizingfield AH is respectively decreased or increased from the value thereforshown in FIG. 2. However, associated with any selected attenuationvalue, the reactive effects attributable to the correspondingvariably-biased film real permeability components will in every caseproduce equal and opposite, canceling dynamic wave phase perturbations.Hence, an incident wave will propagate through the line 10 from thesource 20 to the utilization means 25 with the same relative phasingindependent of the particular wave attenuation produced by the loadedline 10 under control of the energizing winding 40.

In quantitative terms, starting with the field equation 'y is thepropagation constant for the loaded strip line 10,

a is the wave attenuation generated per unit film length by the filmelements 14 or 15,

is the phase shift produced per unit film length by the film elements 14or 15,

w is the angular frequency of the incident wave,

it is defined in Equations l and 2, supra, and

e is the effective permitivity of the medium between the strip linecenter conductor 11 and the outer conductors 12, i.e., the glass 16, itfollows that the wave phase shift per unit film length for each of thefilms 14 or 15 is approximately given by Since n" is always positive forthe films 14 and 15, while p. takes on equal and opposite values, thephase shifts generated by the films 14 and 15 may be seen from Equation4 to be of a like amplitude and opposite polarity. Thus, the FIG. 1arrangement has again been shown by the above to not create any relativephase displacement in -a wave propagating through the attenuating loadedstrip line 10.

It is to be understood that the above-described arrangement is onlyillustrative of the application of the principles of the presentinvention. Numerous other arrangements `may 'be devised by those skilledin the art without departing from the spirit and scope thereof. Forexample, the strip line 10 illustrated in FIG. 1 may be replaced by anywell-known microwave propagating structure, `such as a waveguide orcoaxial cable.

Also, other well-known magnetizing field supplying elements may beemployed to selectively bias the thin film elements 14 and 15 topermeability operating points symmetrically disposed about the filmferroresonant field Ho. Moreover, the film elements 14 and 15 may bepositioned adjacent to the grounded planes 12, rather than contiguous tothe strip line center conductor 11 as shown in FIG. l.

What is claimed is:

1. In combination, means including first and second conductors forpropagating an electromagnetic wave, two ferromagnetic elementsinterposed between said first and second conductors, means for supplyinglike biasing fields to said two elements to bias said elements 'toferroresonance, and means for respectively supplying signal fields of alike amplitude to said two elements, said signal fields having in saidtwo elements aiding and bucking polarities, respectively, with respectto said biasing fields therein.

2. A combination as `in claim 1 wherein each of said ferromagneticelements comprises a ferromagnetic thin film.

3. A combination as in claim 1 wherein said bias field supplying meanscomprises a biasing winding coupled to said two ferromagnetic elementsand a biasing current source serially connected thereto,

4. A combination as in claim 3 wherein said signal field supplying meanscomprises a signal winding coupled to one of said ferromagnetic elementsin the same sense as said biasing winding and coupled to the otherferromagnetic element in a sense opposite to the biasing winding.

5. A combination as in claim 4 wherein each of said ferromagneticelements comprises a ferromagnetic thin film.

6. A combination as in claim 5 further comprising an input wave sourceand an output utilization means each connected to said first and secondconductors.

7. In combination, a wave propagating structure for a signal wave ofpredetermined frequency, rst `and second ferromagnetic means loadingsaid structure, first field supplying means for biasing said firstferromagnetic means to a level above that corresponding to its level forferroresonance at said frequency, and second field supplying means forbiasing said second ferromagnetic means to a level a like amount belowthat corresponding to its level for ferroresonance at said frequency.

8. A combination as in claim 7 wherein said first and secondferromagnetic means comprise ferromagnetic thin film elements.

9. A combination -as in claim 8 wherein said wave propagating structurecomprises a strip line.

10. A combination as in claim 9 further comprising an input wave sourceand an output utilization means each connected to said strip line.

11. A combination as in claim 7 further comprising means forelectronically varying the fields supplied by said first and secondfield supplying means.

12. A combination as in claim 7 wherein said rst and second fieldsupplying means respectively comprise two magnetizing windings coupledto one of said ferromagnetic means in a like sense and coupled to theother of said ferromagnetic means in an opposite sense.

13. The combination in accordance with claim 7 in which said wave ispropagated through said structure in a predetermined direction, and saidferromagnetic means are disposed sequentially along said structure insaid direction.

14. The combination in accordance with claim 13 in which fields suppliedby said field supplying means extend in said direction in said structureand in said `ferromagnetic means.

15. The combination in accordance with claim 13 in which said structureincludes at least one electric conductor, and said Iferromagnetic meansare contiguous to at least one said conductor.

16. The combination in accordance with claim 15 in which saidferromagnetic means comprise ferromagnetic thin film elements.

References Cited by the Examiner UNITED STATES PATENTS 4/ 1964 Brown etal. S33-24.2 9/1964 Petrossian 333-24.3

References Cited by the Applicant UNITED STATES PATENTS OTHER REFERENCESBell System Technical Journal, 1955, Behavior and Applications ofFerrites in the Microwave Region, A. G. Fox et al., Beg. atp. 5 at p.24.

ELI LIEBERMAN, Primary Examiner.

HERMAN K. SAALBACH, P. L. GENSLER Assistant Examiners.

7. IN COMBINATION, A WAVE PROPAGATING STRUCTURE FOR A SIGNAL WAVE OFPREDETERMINED FREQUENCY, FIRST AND SECOND FERROMAGNETIC MEANS LOADINGSAID STRUCTURE, FIRST FIELD SUPPLYING MEANS FOR BIASING SAID FIRSTFERROMAGNETIC MEANS TO A LEVEL ABOVE THAT CORRESPONDING TO ITS LEVEL FORFERRORESONANCE AT SAID FREQUENCY, AND SECOND FIELD SUPPLYING MEANS FORBIASING SAID SECOND FERROMAGNETIC MEANS TO A LEVEL A LIKE AMOUNT BELOWTHAT CORRESPONDING TO ITS LEVEL FOR FERRORESONANCE AT SAID FREQUENCY.